1. Technical Field of the Invention
The present invention relates generally to novel compounds for extracting precious metals from solution, and more particularly to compounds useful for extracting precious metals into supercritical CO.sub.2. Still more particularly, the present invention relates to compounds comprising three part molecules in which the head is a ligand that forms a complex with the desired metal, the body is a spacer group, and the tail is a CO.sub.2 -philic compound that renders the molecule soluble in CO.sub.2.
2. Background of the Invention
In recent years, a growing demand for precious metals in high-technology applications and the increasing cost of precious metals has made recovery of these metals very important. To meet these demands, industry is turning to new sources of precious metals such as complex sulfide ores, and recycling precious metals from catalysts and electronic scrap.
Conventional solvent extraction is sometimes referred to as liquid ion exchange extraction or liquid/liquid extraction. This process comprises two steps. In the first, the extraction step, dilute aqueous feed solution which contains the metal ion to be recovered is contacted with an organic diluent or carrier containing an ion exchanger or ligand dissolved therein. The organic carrier is typically a hydrocarbon and is immiscible in water. The resulting metal complex migrates to the organic phase. In the second, the stripping step, the separated "loaded" organic phase is mixed with an aqueous solution of a stripping agent and the procedure is reversed, with the metal ion passing back to the new aqueous phase. Thus, the dilute feed solution is converted into a highly concentrated solution, from which the metal values are more readily recovered. The barren organic phase can then be recycled through the system.
The system described above has serious drawbacks, however, as the organic carrier is not typically environmentally friendly and the large volumes of water contaminated through contact with the carrier create a sizable disposal problem. Hence it is desired to provide a carrier or solvent that is capable of performing the same extraction without the negative environmental ramifications.
Supercritical carbon dioxide is environmentally innocuous, as well as being inexpensive and safe to handle. Carbon dioxide has elicited significant scientific interest over the past 15 years because it is considered a "green" alternative to conventional organic solvents. CO.sub.2 is inexpensive (approximately $80/ton, 1-2 orders of magnitude less than conventional solvents), non-flammable, and is not currently regulated as a volatile organic compound (VOC).
Although CO.sub.2 possesses distinct advantages as a solvent, it also exhibits three significant disadvantages, which have limited current commercial applications, for the most part, to food processing and polymer foam production. First, use of CO.sub.2 (in either the liquid or supercritical state) requires the use of elevated pressures, as the vapor pressure of CO.sub.2 at room temperature is over 900 psi. Consequently, design and construction of equipment is significantly more expensive than for 1 atmosphere analogs.
Second, utility costs due to processing with high pressure CO.sub.2 can be prohibitively high, in particular those due to gas recompression. Consequently, while it has been suggested that depressurization of a CO.sub.2 solution to 1 atmosphere is an easy route to recovery of products, it is not likely that a CO.sub.2 -based process will be economically viable if extensive depressurization is used to recover dissolved products.
The final significant obstacle to the use of CO.sub.2 as a solvent in conventional chemical processes is its low solvent power. Although its solvent power was once suggested to be comparable to that of liquid alkanes, recent research has shown that this generalization is in error. Calculation produces solubility parameters for CO.sub.2 of 4-5 cal/cm.sup.3 in the liquid state, similar to those of fluorinated materials and slightly lower than those of silicones. It is generally accepted that CO.sub.2 will not solubilize significant quantities of polar, high molecular weight, or ionic compounds. The low CO.sub.2 -solubility of many compounds of interest means that large volumes of CO.sub.2 are required in a potential process, further diminishing the chance for favorable economics.
Hence, it is presently desired to provide an environmentally friendly system for recovering precious metals. The system should be cost-effective and non-hazardous. Thus it is further desired to provide an extractant that is capable of complexing with the desired metal(s) and is soluble in supercritical or liquid CO.sub.2.